Each private key maps to exactly 1 public key, however, there are on average approximately 79,000,000,000,000,000,000,000,000,000 (7.9 X 10²⁸) public keys that map to each bitcoin address.

That being said, you're never going to find any two key pairs that both map to the same bitcoin address.  There are a total of approximately 1,460,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 (1.46 X 10⁴⁸) different addresses and a total of approximately 116,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 (1.16 X 10⁷⁷) private keys.